Various types of apparatus have been used in an attempt to measure the changes in stress forces or pressure within underground formations or other areas such as concrete structures. Most of the prior devices have included the usual strain gauges but as is well known, such strain gauges are rather delicate and do not have a great deal of mechanical strength with the result that any attempts to obtain measurements in areas subjected to relatively high stress changes have resulted in a destruction of the strain gauge device.
Some attempts have been made to construct apparatus of high mechanical strength which will withstand relatively high forces by utilizing a hydraulic system combined with a stress or pressure sensitive element which is exposed to the area under measurement; any stress changes which occur and which act upon the pressure sensitive element are transmitted to the hydraulic system to cause changes in hydraulic pressure which is then converted to a visible indication or record of stress changes.
Although the hydraulic system type apparatus is effective, problems are presented in constructing the apparatus with sufficient sensitivity to provide accurate measurements. To provide sensitivity and accuracy in measuring relatively small changes, the sensing element which is exposed to the area under measurement must necessarily be relatively thin or flexible so as to be readily responsive to minor variations. When so constructed to provide the desired sensitivity difficulty is experienced in attaching this plate to the apparatus in a manner to resist damage or even total collapse under relatively high pressure changes. Therefore, all prior units which have attempted to employ the hydraulic system type of sensing apparatus and which have the necessary sensitivity for obtaining accurate measurements of small variations have lacked overall mechanical strength to withstand high stress changes.